Global green-house and energy concerns have accelerated the electrification of vehicles, which has resulted in a great demand for rechargeable batteries with high energy and power densities. Among various rechargeable batteries for electric vehicles, lithium-ion (Li-ion) batteries have received much attention for their scientific and commercial significance. LiFePO4, one of the most promising cathode materials in Li-ion batteries, has been widely studied owing to its low cost and environmental and safety characteristics. However, the practical application of LiFePO4 was hindered for a long time due to its intrinsically low electronic and ionic conductivity. Good electronic and ionic conductivity are required from an electrode material for the long-term cycling of rechargeable lithium batteries. In response to the considerable challenges of LiFePO4, many research groups proposed to improve its conductivities by reducing the particle size or coating particles with conductive materials such as carbon. The importance of good ionic and electronic conductivity from an electrode material has been proven by LiCoO2, a cathode material widely used in Li-ion batteries which dominate the current electronics market. Therefore, although Li-ion batteries are considered to be a promising system for green transportation, reaching beyond their horizon is a formidable challenge due to the limited energy density which results from the low capacity of LiFePO4 (170 mAh g−1) and LiCoO2 (136 mAh g−1). In comparison, the theoretically significantly higher capacity of sulfur (1,675 mAh g−1) makes it emerge as one of the most promising cathode materials for rechargeable lithium batteries, i.e., lithium-sulfur (Li—S) batteries.
A typical Li—S cell includes sulfur as the positive electrode and lithium as the negative electrode, with a liquid electrolyte as both the charge transfer medium and ionic conductor within the sulfur-containing cathode. Under intense study for more than two decades, there is still no system that works well as a Li—S battery. This is due to the well-known fact that sulfur or sulfur compounds are not only highly electronically insulating but also ionically inactive. Until now, most research has been focused on the synthesis of carbon materials with high surface area for the electronic enhancement of the sulfur cathode. The liquid electrolyte has been used to improve the ionic conductivity of sulfur and its compounds that rely on dissolution in the liquid electrolyte. However, the dissolution of sulfur compounds lead to the polysulfide shuttle, which migrates sulfur species to chemically react with the lithium anode and results in the loss of active materials and poor cycling performance. Though many materials, such as nanostructured carbons, polymers, or graphene composites, were synthesized to retard the diffusion of the bulky polysulfides out of the cathode into electrolyte, the polysulfide shuttle cannot be fully prevented as evidenced by the gradual capacity fading during cycling. There remains a need to improve the intrinsic ionic conductivity of sulfur in Li—S batteries without depending on the liquid electrolyte.